There are various mobile wireless systems using a time division multiple access (TDMA), for instance, EGSM (extended global system for mobile communications) and DCS (digital cellular system) widely used mostly in Europe, PCS (personal communication service) widely used in the U.S., and PDC (personal digital cellular system) used in Japan. According to recent rapid expansion of mobile phones, however, a frequency band allocated to each system cannot allow all users to use their mobile phones in major cities in advanced countries, resulting in difficulty in connection and thus causing such a problem that mobile phones are sometimes disconnected during communication. Thus, proposal was made to permit users to utilize a plurality of systems, thereby increasing substantially usable frequencies, and further to expand serviceable territories and to effectively use communications infrastructure of each system.
Proposed as a small, lightweight high-frequency circuit part capable of handling pluralities of communications systems is a triple-band high-frequency switching module capable of handling three systems of EGSM, DCS and PCS, which is usable for portable communications equipments (WO 00/55983). FIG. 5 is a block diagram showing the triple-band, high-frequency switching module of WO 00/55983, and FIG. 6 is a view showing its equivalent circuit, though the reference numerals of control terminals are changed for convenience. This high-frequency switching module for switching three transmitting/receiving systems comprises (a) a diplexer comprising first and second filter circuits F1, F2 for viding a signal received by an antenna ANT to a receiving signal of the first transmitting/receiving system and a receiving signal of the second and third transmitting/receiving systems, (b) a first switching circuit SW1 downstream of the first filter circuit F1 for switching a transmitting circuit TX1 and a receiving circuit RX1 of the first transmitting/receiving system by voltage applied from a control circuit VC1, and (c) a second switching circuit SW2 disposed downstream of the second filter circuit F2 for switching a transmitting circuit TX2 of the second and third transmitting/receiving systems, a receiving circuit RX2 of the second transmitting/receiving system and a receiving circuit RX3 of the third transmitting/receiving system by voltage applied from control circuits VC2, VC3. This reference exemplifies a case where the first communications system is EGSM (transmitting frequency: 880-915 MHz, receiving frequency: 925-960 MHz), the second communications system is DCS (transmitting frequency: 1710-1785 MHz, receiving frequency: 1805-1880 MHz), and the third communications system is PCS (transmitting frequency: 1850-1910 MHz, receiving frequency: 1930-1990 MHz). The operation of the first to third control circuits and diode switches will be explained below.
(A) DCS/PCS TX Mode
When the second/third transmitting circuit TX2 is connected to the second filter circuit F2, a positive voltage is applied from the control circuit VC2, and a zero voltage is applied from the control circuit VC3. The positive voltage applied from the control circuit VC2 is deprived of a DC component by capacitors CP2, CP3, CP4, CP5, CP6 and CF4 and applied to a circuit comprising diodes DP1, DP2, so that the diodes DP1, DP2 are turned on. When the diode DP1 is turned on, impedance is lowered between the second/third transmitting circuit TX2 and the input/output terminal IP2. The turned-on diode DP2 and capacitor CP6 make the transmission line LP2 grounded at high frequencies and thus resonated, extremely increasing the impedance of the output terminal IP3 viewed from the input/output terminal IP2. Further, the turned-off diode DD2 increases impedance between the output terminal IP3 and the third receiving circuit RX3. As a result, a transmitting signal from the second/third transmitting circuit TX2 is transmitted to the second filter circuit F2 without leaking to the second and third receiving circuits RX2, RX3.
(B) DCS RX Mode
When the second receiving circuit RX2 is connected to the second filter circuit F2, a zero voltage is applied from the control circuits VC2, VC3, so that the diodes DP1, DP2, DD1 and DD2 are turned off. The turned-off diode DP1 increases impedance between the input/output terminal IP2 and the second/third transmitting circuit TX2. Also, the turned-off diode DD2 increases impedance between the output terminal IP3 and the third receiving circuit RX3. The input/output terminal IP2 is thus connected to the second receiving circuit RX2 via the transmission lines LP2 and LD1. As a result, a receiving signal from the second filter circuit F2 is transmitted to the second receiving circuit RX2 without leaking to the second/third transmitting circuit TX2 and the third receiving circuit RX3.
(C) PCS RX Mode
When the third receiving circuit RX3 is connected to the second filter circuit F2, a positive voltage is applied from the control circuit VC3, and a zero voltage is applied from the control circuit VC2. The positive voltage from the control circuit VC3 is deprived of a DC component by capacitors CDP1, CDP2, CDP3 and CP5, and applied to a circuit comprising the diodes DD1, DD2, so that the diodes DD1 and DD2 are turned on. The turned-on diode DD2 decreases impedance between the third receiving circuit RX3 and the output terminal IP3. Also, the turned-on diode DD1 and capacitor CDP2 make the transmission line LD1 grounded at high frequencies and thus resonated, extremely increasing the impedance of the second receiving circuit RX2 viewed from the output terminal IP3. Further, the turned-off diode DP1 increases impedance between the input/output terminal IP2 and the second/third transmitting circuit TX2. As a result, a receiving signal from the second filter circuit F2 is transmitted to the third receiving circuit RX3 without leaking to the second/third transmitting circuit TX2 and the second receiving circuit RX2.
(D) EGSM RX Mode
When the first receiving circuit RX1 is connected to the first filter circuit F1, a zero voltage is applied from the control circuit VC1 to turn off the diodes DG1 and DG2. With the diode DG2 turned off, the input/output terminal IP1 is connected to the first receiving circuit RX1 via the transmission line LG2. The turned-off diode DG1 increases impedance between the input/output terminal IP1 and the first transmitting circuit TX1. As a result, a receiving signal from the first filter circuit F1 is transmitted to the first receiving circuit RX1 without leaking to the first transmitting circuit TX1.
(E) EGSM TX Mode
When the first transmitting circuit TX1 is connected to the first filter circuit F1, a positive voltage is applied from the control circuit VC1. The positive voltage is deprived of a DC component by capacitors CG6, CG5, CG4, CG3, CG2 and CG1, and applied to a circuit comprising the diodes DG2 and DG1, so that the diodes DG2 and DG1 are turned on. The turned-on diode DG1 decreases impedance between the first transmitting circuit TX1 and the input/output terminal IP1. The turned-on diode DG2 and capacitor CG6 make the transmission line LG2 grounded at high frequencies and thus resonated, extremely increasing the impedance of the first receiving circuit RX1 viewed from the input/output terminal IP1. As a result, a transmitting signal from the first transmitting circuit TX1 is transmitted to the first filter circuit F1 without leaking to the first receiving circuit RX1.
The above control logic is summarized in Table 1. Thus, one mode of the first to third transmitting/receiving systems is selected by controlling the ON/OFF states of diodes in the switching circuit by voltage applied from control circuits.
TABLE 1ModeVC1VC2VC3EGSM TX (Transmitting)HighLowLowDCS/PCS TX (Transmitting)LowHighLowEGSM RX (Receiving)LowLowLowDCS RX (Receiving)LowLowLowPCS RX (Receiving)LowLowHigh
In the high-frequency switching module, insertion loss is preferably as small as possible, because it affects the battery lives of cell phones in a transmitting mode, and receiving sensitivity in a receiving mode. Harmonics are also preferably as small as possible to reduce unnecessary power consumption in each system. It is particularly important to limit a second harmonic. Attenuation is preferably, for instance, −35 dB or more in EGSM and −25 dB or more in DCS/PCS. Various measures have conventionally been taken to suppress harmonics, but a measure conducted at present is to optimize the characteristics of a lower-frequency filter in a diplexer or a transmitting lowpass filter in a high-frequency switching module, thereby improving the attenuation by several tenth of dB. However, such measure has limitations, failing to obtain higher attenuation of harmonics than the above level.